This invention relates to a deflection yoke which includes a raster rotation coil, and more particularly to a deflection yoke which allows compatibility of correction of raster distortion other than raster rotation performed using a raster rotation coil and correction of misconvergence performed by producing a difference in current between an upper side horizontal deflection coil and a lower side horizontal deflection coil.
Some deflection yokes for a color cathode ray tube include a raster rotation coil. A raster rotation coil is frequently located on the front side of a deflection yoke, that is, adjacent to a screen with respect to a cathode ray tube.
FIG. 1 is a perspective view showing an entire deflection yoke which includes a raster rotation coil, and FIG. 2 is a side elevational view showing only coils and a DY core of the deflection yoke of FIG. 1.
A pair of upper and lower saddle type horizontal deflection coils 3 are wound around a horizontal deflection coil bobbin 1. A pair of left and right saddle type vertical deflection coils 7 are wound around a vertical deflection coil bobbin 5 on the outer sides of the horizontal deflection coils 3.
A DY core 9 made of ferrite is mounted such that it surrounds the vertical deflection coils 7. An annular (circular, polygonal or the like) raster rotation coil 11 is mounted along an outer periphery of the deflection yoke on the front side of the deflection yoke. It is to be noted that, in FIGS. 1 and 2, reference numeral 13 denotes an adjustment knob, 15 an adjustment coil, 17 a rear cover, and 19 a front cover.
The raster rotation coil 11 has a winding of several hundreds turns of a nylon-coated wire of a diameter of approximately .o slashed. 0.2 to .o slashed. 0.4 mm.
The raster rotation coil 11 is usually used in order to correct rotation of a raster appearing on the screen of the cathode ray tube around an axis of the tube.
For example, if a location of a receiver in which a cathode ray tube is used is varied, then the influence of the terrestrial magnetism upon the cathode ray tube varies. This sometimes rotates the raster in a fixed direction into an inclined state.
In such an instance, the raster in an inclined state is corrected by DC current flowing in the raster rotation coil.
In addition, a magnetic field generated by the raster rotation coil interferes with the deflection coil. However, if fixed DC current flows in the raster rotation coil, then also the magnetic field is fixed and it does not occur that induction current flows in the deflection coil as a result of the interference. Consequently, a convergence characteristic is not influenced by the interference.
If a triangular wave current having a period equal to a vertical deflection period or an arbitrary period or an alternating current of an arbitrary waveform is supplied to the raster rotation coil 11, then also the amount of rotation of the raster varies in response to the amount of the current. This allows correction of a distortion other than raster rotation, for example, correction of trapezoid distortion or the like of a raster. This technique is effective to products for which a severe quality in regard to raster distortion is required, for example, to display units for computers.
However, since the magnetic field generated by the raster rotation coil varies with respect to time, induction current flows in the deflection coil by an influence of the temporal variation of the magnetic field. This current causes a variation of the convergence characteristic and gives rise to so-called misconvergence.
A countermeasure against this problem has been proposed already. Referring to FIG. 3, a pair of coils 21 wound on a shared core are respectively connected in series to two upper and lower horizontal deflection coils 23 and 25. The pair of coils have a strong magnetic connection since they share the core.
In the deflection yoke 27 having such a construction as described above, the coils 21 connected in series to the two upper and lower horizontal deflection coils 23 and 25 have high inductance values only for induction current generated by a variation of the magnetic field of the raster rotation coil.
For example, if triangular wave current flows through the raster rotation coil 11, then induction current flows in the deflection coils. The induction current is suppressed by the coils 21 connected in series to the upper side horizontal deflection coil 23 and the lower side horizontal deflection coil 25.
Consequently, little influence is on the deflection current flowing in the circuitry of the receiver, and only the induction current induced by a variation of the magnetic field of the raster rotation coil is suppressed and the variation of convergence can be reduced. Since the coils 21 have a function of suppressing induction current, they are called induction current suppressing coils.
However, a series connection of such two coils having a strong magnetic connection as described above to upper and lower horizontal deflection coils gives rise to a new problem. In particular, if a vertical assembly error remains between a cathode ray tube and a deflection yoke, then this causes vertical asymmetry between an electron beam of the cathode ray tube and a horizontal deflection magnetic field. For example, if the horizontal deflection magnetic field is displaced downwardly with respect to the electron beam as shown in FIG. 4A, then misconvergence as shown in FIG. 4B appears on the screen.
As a popular method of correcting the misconvergence, variable coils are connected in series to upper and lower side deflection coils as shown in FIG. 4C. The variable coils are used in order to adjust the balance in inductance value, and therefore called balance coils.
If the inductance value of a balance coil increases, then the current flowing therethrough decreases, but if the inductance value decreases, then the current increases. By adjustment of the balance coils, a difference can be provided between the amounts of deflection current to be supplied to the upper and lower side horizontal deflection coils.
Vertical asymmetry of the magnetic field can be adjusted by the difference in current. Accordingly, vertical asymmetry of the deflection current can be corrected as shown in FIG. 4D, and the misconvergence illustrated in FIG. 4B can be corrected.
However, in the circuit shown in FIG. 3, the induction current suppressing coils 21 connected in series between the upper and lower side horizontal deflection coils and the upper and lower side balance coils have a strong magnetic connection and are connected in the opposite phases to each other.
Therefore, even if it is tried to adjust the current between the upper and lower side horizontal deflection coils, the induction current suppressing coils 21 act to cancel current variations of the upper and lower side horizontal deflection coils.
Accordingly, for a deflection yoke for which a coil for suppressing induction current is mounted, the method of correcting misconvergence using balance coils which are used popularly cannot be used.